The present invention relates to an outsole construction for a shoe. More particularly, the present invention relates to a construction which provides all midsole and outsole functions for a shoe with a single homogeneous moldable material through geometry alone. The present invention employs cut-out geometry for engineering various characteristics into an outsole of a uniform, monolithic material. These characteristics are obtained by removing material or reinforcing the material so that it functions as if it had different densities.
In an attempt to understand the foot as a system, the various parameters which affect the function of the foot have been studied, particularly with regard to a weight bearing foot. The practical need for such knowledge lies in the fact that a true structural model of the foot is capable of providing a prediction of gait and the effect of a shoe on gait. By knowing, in advance, how a shoe would affect the performance of an athlete, for example, optimum shoes could be designed without the usual "cut and try" method of standard shoe development.
The traditional model of the foot provides for a one column, two-axis model which maintains that the foot under load is a rigid structure with a talocrural (ankle) axis and an apparent subtalar axis. The front of the foot is relatively rigid, but with only a multitude of small bone movements about the midtarses axes. The average direction of the effective axis under the ankle, called the subtalar axis, is said to be 42 degrees vertical and 16 degrees horizontal to the midline of the body, as measured by Inman, V.T., The Joints of the Ankle, The Williams & Wilkins Co., Baltimore, 1976. However, this theory does not hold up with regard to a weight bearing or loaded foot since, if the force due to body weight were to act on the single traditional subtalar axis, the foot would collapse mechanically.
It has now been determined that the foot is comprised of two columns and three axes. The lower, lateral column is basically a rigid base comprised of the Calcaneus, Cuboid, and the fourth and fifth metatarsals. The remainder of the foot, which is comprised of the navicular, the first, second and third cuneiforms and the first, second and third metatarsals, emanates from the talus at the talonavicular interface swinging in combination with the lower column inversion/eversion actions in what may be called the `subtalar joint axis`. But this articulation of what is called the upper foot column is only secondary to the true foot mechanism. The primary mechanical loading interface is on the lower, lateral column at the rear of the talus onto the calcaneus, the posterior talocalcaneal facet.
It has also been determined that the foot operates differently under load than when it is passively manipulated such as a doctor would do in the office. This distinction helps to explain previous misconceptions as to how the foot works under load.
This new understanding has yielded a new structural model of the foot which has two separate columns, wrapped together with fascia, and three nearly orthogonal axes. The three axes are: (1) the talocrural (ankle) axis; (2) the talocalcaneal axis (formed at the facet between the talus and the calcaneus); and (3) the talonavicular axis (formed at the facet between the talus and the navicular bones).
There have been molded shoe outsoles in existence for many years but such constructions have been intended primarily to deal with problems of cushioning, tread and traction, and to mate with a lasted shoe upper and to be affixed thereto with adhesives and/or stitching. Such previous constructions have not been intended to provide an optimal base for the structural human foot. In particular, such constructions have not been based on a two column structural load frame as described herein. In addition, most previous outsole constructions are not designed to accommodate the change in function which occurs with only a small amount of wear of the edges on the bottom surface of the outsole.
By the present invention, there is provided an improved outsole construction in which, starting with a monolithic, thin, relatively soft, tough elastomer, all functions known to be needed by the structural foot model are addressed by addition or subtraction of material. The present invention is based on the principle of "minimum sufficient thickness" in order to maintain the foot as close to the ground as possible.
The outsole shape is what is commonly called "in-flared" and has a detailed outline which is sufficient to support more than 90 percent of the foot population for a given foot length.
In the outsole construction of the present invention, energy absorption/cushioning is achieved by the following features:
1. A series of transverse slots along the lateral border to provide cushion conformability to the lower foot column.
2. An array of compression columns or holes in the heel region to accept the known pressure distributions.
3. An array of small holes in the ball region to add both cushioning and flexibility.
4. A raised heel cup to constrain the heel fat pad expansion during impact and advantageously enhance the natural shock absorbing characteristics of the foot.
5. Relief of the rear lateral heel border to dynamically smooth and cushion initial heel strike.
Static and dynamic stability in the present outsole construction are achieved by the following features:
1. Complete material support for the entire foot structure weight bearing points.
2. Minimum thickness and maximum flexibility to reduce any inversion/eversion torques including ankle sprains.
3. Heel cupping to constrain calcaneal movement.
4. Effective radial heel to reduce excessive eversion torques about the talocalcaneal axis, dynamically.
5. Subtle lateral border chock to reduce inversion rollover during standing.
6. Complete forefoot flexibility for firm footing on any pitched or irregular surface.
7. Firm toe base for gripping in balance and toe-off.
In the use of the outsole construction of the present invention, the "two column" foot has complete freedom of rotational motion because of transverse and longitudinal outsole flexibility. This is due to the thinness and softness of the outsole as well as relief of material to aid with this flexibility. In addition, exceptional durability is achieved by eliminating local wearing forces and rotations with the use of firm, flexible footing throughout the entire gait cycle. Also there are no materials which will degrade under repeated impacts and flexures.
The outsole construction of the present invention allows the three dimensional geometry of the shoe upper to be referenced precisely to the foot base or outsole at accurately placed anchor points without the use of a last.
The construction of the present invention also mechanically holds the shoe to the foot in the midfoot region where a support sling construction may be anchored.
Accordingly, it is a primary object of the present invention to provide all midsole and outsole functions for a shoe with a single homogeneous moldable material through geometry alone.
It is another object of the invention to provide anchor sites for a transverse support sling having straps which are carried over the top of the midfot and allowed to fan out with attachment to the outsole under the upper, medial column of the foot so as to optimally support the upper column when the foot is loaded.
It is a further object of the present invention to provide reference anchor points for upper attachment to eliminate the need for a shoe last in manufacturing.
It is another object of the invention to create a midsole/outsole construction which will maintain its functional performance for the reasonable life of the shoe.